1. Technical Field
This invention generally relates to semiconductor testing, and more specifically relates to testing and debugging semiconductor chips.
2. Background Art
The fabrication of integrated semiconductor devices involves forming a plurality of devices on a semiconductor wafer. The wafer is then divided up into a plurality of pieces called "chips" with each chip comprising one or more semiconductor devices. Each chip is then placed in a package that has external connections to provide accesses to signals on the chip.
Many different packages for integrated circuits have been developed. One popular package is the dual in-line package (DIP). The DIP is commonly a plastic package for many commercial applications, but also comes in ceramic packages for applications that require higher operating temperature. As the number of pins on a chip were increased, new packages were developed, such as the pin-array (PGA). A PGA package typically has rows and columns of pins in an array, and may be either plastic or ceramic as well. As the size of electronic boars continues to shrink, other packages have been developed that provide a higher density of connections in a given space.
For example, surface-mount devices have been developed that have a ceramic package (or module) with solder pads that provide connections to the integrated circuit. These solder pads may be made much smaller than the area required by a pin in prior art DIP and PGA packages, resulting in a higher connection density.
Surface mount modules may be mounted on a variety of different types of circuit boards, circuit modules, or other substrates, referred to herein generically as a "system board". A system board designed to receive a surface mount module typically provides landing pads that align with the landing pads on the module. Solder balls or solder bumps may be formed on either the module landing pads, the system board landing pads, or both. The surface mount module is then placed on the system board and the entire assembly is heated until the solder balls flow and form a good electrical connection between landing pads. The array of solder balls thus serve as an interconnect mechanism between the landing pads on the module and the landing pads on the system board.
As an example of surface mount modules, ball grid array (BGA) and column grid array (CGA) chip carrier modules have used arrays of solder balls or columns (sometimes referred to as cylinders) as input and output connections. In this application, the term "solder balls" will be used generically to refer to the balls, bumps, columns, cylinders or other suitable connections used as surface mount module interconnects. Generally, the array of solder balls are arranged on a dense pitch of 1.0 and 1.27 millimeters. With a dense array of solder balls covering one side of the module, BGA and CGA modules can provide a large number of input and output connections to the chip in the module without using excessive space.
In BGA and CGA modules the array of solder balls and columns are connected to the chips in the modules by a corresponding array of landing pads on the surface of the chip. These landing pads are generally part of the last metal layer interconnect from the semiconductor device, and comprise metal such as aluminum (Al) and copper (Cu). These solder balls generally comprise a lead tin alloy (PbSn) or other soft metal.
When the modules are connected to the system board the modules are flipped over and placed so that the array of solder balls are aligned with the corresponding array of landing pads on the system board. The module and system board are then heated, allowing the solder paste, which is screened on an array of landing pads, to melt and flow into the system board. This establishes the physical and electrical connection between the module and the system board.
Unfortunately, the dense array of solder balls makes testing the modules very difficult. The difficultly is increased where the module must be attached to another device, such as a printed circuit board, for testing. For example, some modules must be attached to the system board before they can be completely tested. Testing these modules requires access to the solder balls be provided while the module is attached to the system board. Unfortunately, it is especially difficult, and in some cases impossible, to probe the input/output solder balls on the module when it is attached to the system board because the body of the module and the system board block access to the solder balls due to the surface mount attachment.
In the past, one way designers used to test and debug CGA and BGA modules was to form testing contact pads into the system board that provide contact points to the solder balls nodes. Unfortunately this requires additional real estate on the system board, an unacceptable solution in many cases. Additionally, these types of landing pads generally requires a specialized and expensive probing tool with a long lead time required to build it.
Another method designers have used to debug and develop CGA and BGA packaged devices was to package them in a simplified package such as a pin grid array package (PGA), with the input/output connects through standard pin outs and then design and build a specialized test system board that is used only for debugging and testing. This specialized test system board would be similar to the normal system board, but with the design changes necessary to use the PGA modules instead of the BGA or CGA module. The designer could then use an interface card to connect to the specialized test system board to the overall system for testing.
The specialized test system board would be specially designed to provide landing pads for test probes that can be used to provide access to the solder balls and connect to a logic analyzer and/or other measurement devices. Thus, the designer was able to access the pins and solder balls on the CGA or BGA module and test the module while it is connected to the overall system through the test PCB. Unfortunately, designing and building a specialized PGA package and a specialized test system board simply for the testing of the module was exceedingly time consuming and expensive. In particular, it requires the design and development of two packages, a test PGA package and a CGA or BGA module used for final packaging. Additionally, it requires special boards to be designed. The specialized test system board with its extra landing pads for testing and the normal system board without the landing pads that will be used in the final product.
Thus, what is needed is a method and apparatus to provide debugging and testing of BGA/CGA modules while on the system board or other packaging that does not require the specialized test packages or specialized test system boards, and does not require the use of valuable real estate on the system board for testing pads.